1. Field of the Invention
The present invention relates in general to an image projecting apparatus such as a rear-type liquid-crystal projector and in particular to correction of color non-uniformity of a projected image in an image projecting apparatus.
2. Description of the Related Art
In the related art image projecting apparatus such as a rear-type liquid-crystal projector, a light passing through a liquid-crystal light valve is projected on a screen for displaying an image. To put it in detail, an illumination light comprising red (R), green (G) and blue (B) color components is generated by a light source such as a metal halide lamp. The illumination light is passed through spectroscopic means to produce R, green G and blue B illumination lights. The R, G and B illumination lights then pass through their respective liquid-crystal light valves which are driven by using red, green and blue color signals. After the R, G and B illumination lights passing through the respective liquid-crystal light valves are synthesized by using a synthesizing means, they are projected on a screen from the rear side thereof by a projection means, forming a color projected image on the screen.
FIG. 1 is a diagram showing a typical configuration of the related art rear-type liquid-crystal projector. As shown in the figure, a lamp 1 radiates a light comprising red (R), green (G) and blue (B) color components. The light radiated by the lamp 1 is applied to fly eye lenses 2 and 3 which serve as an optical integrator. The fly eye lenses 2 and 3 focus a light generated by a lamp before radiating the focused light to a liquid-crystal light valve uniformly as shown in FIG. 2, a diagram showing the operation of fly eye lenses.
In actuality, the R color component passing through the fly eye lenses 2 and 3 is reflected by an R dichroic mirror 4 and a mirror 5, being directed to an R liquid-crystal light valve 7 through a condenser lens 6 for converting the R color component into all but parallel beams as shown in FIG. 1.
By the same token, the G color component passing through the R dichroic mirror 4 is reflected by a G dichroic mirror 8, being directed to a G liquid-crystal light valve 10 through a condenser lens 9 for converting the G color component into all but parallel beams.
After leaving the G dichroic mirror 8, the remaining B color light component passes through a relay lens 11, a mirror 12 and a relay lens 13 and is reflected by a mirror 14. The B color component is then converted by a condenser lens 15 into all but parallel beams which are applied to a B liquid-crystal light valve 16.
In each of the R liquid-crystal light valve 7, the G liquid-crystal light valve 10 and the B liquid-crystal light valve 16 which are driven by their respective video signals, an optical image is formed as variations in transmittance due to a video signal applied thereto. Lights output by the R liquid-crystal light valve 7, the G liquid-crystal light valve 10 and the B liquid-crystal light valve 16 are synthesized by a dichroic prism 17 into a single light, substantially forming a color image at the position of the G liquid-crystal light valve 10.
The dichroic prism 17 is a prism-type dichroic mirror built by joining four rectangular prisms 17a, 17b, 17c and 17d. An R reflective dichroic multi-layer film and a B reflective dichroic multi-layer film are deposited on junction surfaces 19 and 20 inside the dichroic prism 17 respectively.
The color image synthesized by the dichroic prism 17 is enlarged by a projection lens 18, being projected on a screen, which is not shown in the figure, from the rear side thereof.
In the related art configuration described above, however, the light reaching the dichroic prism 17 has a predetermined incident inclination because the light has been converged before being radiated to the dichroic prism 17. For example, R light beams applied to the dichroic prism 17 shown in FIG. 3A, a diagram showing an enlarged view of the dichroic prism 17 as seen from a position above the prism 17, form different incident angles .theta.0, .theta.1 and .theta.2 with normal lines at the center, an upper portion and a lower portion of the junction surface 19 respectively as shown in FIG. 3B, a diagram showing incident angles of red light beams entering the dichroic prism 17. On the other hand, B light beams applied to the dichroic prism 17 shown in FIG. 3A form different incident angles .theta.0', .theta.1' and .theta.2' with normal lines at the center, an upper portion and a lower portion of the junction surface 20 respectively as shown in FIG. 3C, a diagram showing incident angles of blue light beams entering the dichroic prism 17.
The dichroic multi-layer films deposited on the junction surfaces 19 and 20 inside the dichroic prism 17 each exhibit dependence on the incident angle, that is, a reflectance for a light which varies in dependence upon the incident angle of the light. As a result, when light beams are applied to the dichroic multi-layer film at different incident angles as described above, wavelength shifts are inadvertently generated as shown in FIG. 3D, a diagram showing characteristics of the dichroic prism 17 with respect to an S polarized light. As shown in FIG. 3D, with respect to the incident angle .theta.0, the wavelength is shifted to shortwave and longwave sides at the incident angles .theta.2 and .theta.1 respectively. As for the incident angle .theta.0', the wavelength is shifted to shortwave and longwave sides at the incident angles .theta.1' and .theta.2' respectively.
The wavelength shifts occurring in the dichroic prism 17 considerably deteriorate the color purity of the liquid-crystal projector employing the dichroic prism 17 in the configuration thereof. When a red color is displayed on a screen, for example, a color close to a pure red color is displayed on the right half of the screen while color shading occurs on the left half, resulting in an orange color as shown in FIG. 4A, a diagram showing a front view of a display screen.
Such deterioration of a color purity has an effect on the luminance distribution of the displayed image as shown in FIG. 4B, a diagram showing a luminance distribution on a display screen. As shown in the figure, the left half of the screen exhibits bright luminance variations in comparison with the right half. For this reason, it is necessary to make the luminance distribution uniform by insertion of typically an attenuation filter for the left half of the screen, giving rise to a problem that such a filter deteriorates the efficiency of the utilization of the light in the liquid-crystal projector.
As a technique to solve such a problem, the thickness of the dichroic multi-layer film deposited on the junction surfaces 19 and 20 inside the dichroic prism 17 is changed in accordance with the incident angle of the light applied thereto. However, such a technique entails a complicated structure of the dichroic prism 17. In addition, introducing much waste of materials, the technique is economically undesirable.